Secondary Treatment for Carbonaceous Biochemical Oxygen Demand (CBOD) removal is provided by the activated sludge process in covered reactors in combination with final sedimentation tanks. Sewage enters the reactors in a plug flow pattern and is combined with return sludge to form mixed liquor. Mechanical aeration equipment is provided to transfer high purity oxygen to the mixed liquor in the reactors. Pure oxygen is generated on site with one 80-ton air separation plant.
In the secondary sedimentation tanks, the solids (activated sludge) are removed from the mixed liquor by gravity settling. The solids may then be returned to the reactors or wasted from the process as required. There is a greater than 90 percent removal of carbonaceous CBOD and suspended solids. Also, between 15 and 20 percent of the nitrogen is removed along with the suspended solids.